1. Field of the Invention
The present invention relates to a network system and to a network control method by which a data rate can be changed.
2. Discussion of Related Art
In a general network in which data are transmitted, it is desirable to vary the data rate according to the condition of the network. For example, not all devices connected to the network are able to communicate using the same data rate, some older devices may have to use a slower data rate than newer devices. Furthermore, the data rate could be changed according to the load on the network. Another reason for changing the data rate is a connection to a second network in which another data rate is used.
Especially in case of a mobile telecommunication network, changing of the data rate should be effected such that the data transmission is not disrupted. Otherwise, the data rate change would worsen the quality of service.
Recently modems have been proposed which are able to effect a seamless data rate change. That is, such modern modems, e.g., ITU-T V.34, can (re)negotiate the data rate, i.e., upgrade and downgrade the data rate, during the call. This feature is useful in the beginning of the call in order to adapt to the prevailing conditions, or even during the call to optimize the throughput by adapting to changing conditions.
ITU-T modem recommendation V.34, annex A, describes a Seamless Rate Change method. In this method the modems change the line rate without disrupting the data transmission for training sequences. The modems negotiate the new rate through a low rate inband control channel.
The impact of the change of line rate on the terminal is at its best just an increased or decreased use of flow control in the data terminal equipment/data communications equipment (DTE/DCE) interface.
In case a change of the data rate is caused from an external element, i.e., an element outside the network concerned, there occurs the problem that the data rate can not be smoothly changed. This is especially a problem when a call is transmitted from one network to another.
An example for a network system in which this problem can occur is shown in FIG. 1.
Reference numeral 1 denotes a radio access network (RAN). This network can be a GSM network or a UMTS network, for example. Reference numeral 2 denotes a mobile station MS which is connected over an air (radio) interface with a base station BS 3 of the radio access network (RAN) 1. The radio access network 1 is controlled by a mobile services switching center (MSC) 4. The MSC 4 controlling the base station 3 comprises an Interworking Function (IWF) controller which performs communication with a second network 5, which is in this embodiment a fixed network, e.g., a Public Switched Telephone Network (PSTN). The fixed network 5 comprises a PSTN network controller 6 in which a modem is included which will be referred to as PSTN modem in the following.
For such a network system, transparent bearer services are defined to support a constant data rate end to end. In a 3.1 kHz audio (=modem) case, this means that the data rate in the GSM traffic channel (between the MS 2 and the MSC IWF 4) and in the PSTN network leg (between the MSC IWF 4 and the PSTN controller) are the same. If this were not the case, data would be lost (due to a buffer overflow) or duplicated (due to a buffer underflow) in the MSC IWF modem.
It is necessary that in both legs data are transmitted using the same data rate. Hence, if in such a case the data rate is to be changed, the quality of service and transmission is affected by this change, since due to changing of the data rate, the data transport can be discontinued, even in the case when the MS 2 and/or the MSC 4 comprise modems which are able to perform a seamless rate change as described above.